In size the teeth of _H. tenuiceps_ average twenty per cent larger than any of the upper teeth of _H. gregoryi_, _H. hatcheri_, or the Pipestone Springs specimen, and exceed any of the known lower teeth including those of _H. vetus_ and _H. senex_ by twenty-five per cent or more. Inasmuch as the upper teeth rarely exceed the lower in length in all the related genera of heteromyids, it is assumed that a similar relationship existed between the upper and lower molars of _H. tenuiceps_ and, therefore, that this species can be distinguished by its large size. The relative size of the premolars and molars is the same in _H. tenuiceps_ as in other species of _Heliscomys_. However, within the framework of this similar relationship there are two differences. P4 of _H. tenuiceps_ is relatively larger than the P4 of _H. gregoryi_, and relatively smaller than the P4 of _H. hatcheri_. The width of the molars is relatively greater in _H. tenuiceps_ and _H. gregoryi_ than in _H. hatcheri_. MEASUREMENTS (In millimeters) U. K. M. N. H. (Vert. Paleo.) No. 7702 Height of skull at M2 7.48 Length from anterior end of nasals to rear of M3 15.41 Length of nasal bones 10.50 Width of rostrum at highest point of root canal 3.97 Interorbital width 4.39 Estimated length of skull 25.00 I, anteroposterior length 1.56 I, transverse width 0.63 P4-M3 crown length 3.75 P4-M3 alveolar length 3.80 P4, anteroposterior length[A] 1.05 P4, transverse width 1.08 M1, anteroposterior length 0.93 M1, transverse width 1.17 M2, anteroposterior length 0.93 M2, transverse width 1.14 M3, anteroposterior length 0.78 M3, transverse width 0.93 [Note A: This and the following measurements at occlusal surface.] _Discussion._--_Heliscomys tenuiceps_ shows beyond any doubt that the heteromyid pattern of skull was developed by mid-Oligocene times, and in this species was already undergoing lateral compression. The major change later made in heteromyid skulls is broadening of the dorsal surface of the skull in the interorbital area.